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On Wed, 31 Oct 2007 16:56:23 +0000, Phil Cook wrote:
> And lo on Tue, 30 Oct 2007 14:54:20 -0000, Jim Henderson
> <nos### [at] nospam com> did spake, saying:
>
>> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
>>
>>> Then wikipedia is horribly wrong, I suppose.
>>
>> Wikipedia isn't always right....
>
> Blasphemer! May a thousand sysops poop on your head :-)
Ah, well, that's already happened. (I was a SysOp on CompuServe years
ago <g>)
Jim
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Jim Henderson escribió:
> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
>
>> Then wikipedia is horribly wrong, I suppose.
>
> Wikipedia isn't always right....
Then edit it.
I hate people saying "don't use wikipedia because it's wrong". Make it
right instead of whining!
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>> Friction against what? Newton's 3rd law...
>
> Against the ground
Equal and opposite torque on the person and the Earth ... angular momentum
is still conserved.
> and the air.
Same for the air - assuming you take the air as being part of the "Earth".
If not, then well you've got an external force on the system and angular
momentum will not be conserved (but at some point the air will have some
other effect on the Earth that will cancel out, the wind can't keep getting
faster and faster for ever).
> Friction causes heat, and the energy for that heat must come from
> somewhere.
> I cannot be produced from nothing.
The heat is produced from the energy you need to maintain the force over
time (ie work done), eg from your food that you use to push your legs, or
the electricity in some electric motor. Without that there would be no
force against the frictional force and the spinning would eventually stop.
Momentum (both angular and linear) is ALWAYS conserved in a system with no
external forces, always always always. It's simply impossible to change the
momentum of a system without applying some external force (or torque).
Energy is also always conserved, but you have to consider ALL the energy
going into and out of the system, including heat, kinetic, any potentials in
gravitational/magnetic fields, chemical etc. Look up the laws of
thermodynamics for further info.
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"Warp" <war### [at] tagpovrayorg> wrote in message
news:472379eb@news.povray.org...
> Ok, here's an easy way of stopping the rotation of an object in a closed
> system:
>
> Construct a cannon on the surface of the object so that it will shoot
> in the direction of the tangent, towards the rotation direction. Shoot
> a heavy-enough object fast enough so that the recoil of this shot will
> stop the rotation of the main object. (If you are worried about possible
> effects of having just one such cannon on the surface of the object then
> just put another identical cannon at the opposite side of the object,
> aimed towards the opposite direction, ie. still at the direction of
> rotation at that side, and fire both cannons simulatenously.)
>
> The projectile is attached to a cable connected to the object at a
> point where the pull caused by the object when it reaches the cable
> length causes an even force on the center of mass of the object. (Such
> a point must exist because there are two extremes: Attaching the cable
> on the object at the point where the cannon is would effectively negate
> the whole operation. However, attaching the cable on the opposite side
> of the object would actually make it start rotating in the opposite
> direction compared to the original one. A balance point must thus exist
> somewhere in between.)
>
> When the projectile has thus been stopped, just pull it back to the
> surface of the object. The rotation of the whole system will have stopped
> without permanently ejecting any material.
>
> (Where did all the angular momentum go? Well, stopping the projectile(s)
> will require energy. The majority of this energy will be dissipated as
> heat from the cable.)
This violates one of the fundamental principles of dynamics, the principle
of conservation of momentum.
When you "pull it back", this will cause the object to start spinning again,
and you'll find that when you have the projectiles back where they started,
the spin will be exactly the same as when you started.
The energy you lost in friction/heat will be identical to the energy you
used to fire the cannons in the first place.
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> So you are effectively saying that regardless of heat produced by
> friction, angular momentum is always conserved.
Yes.
> This would effectively
> make a spinning object an infinite source of energy.
Why infinite? You slow it down until it stops, then how do you propose to
get out any more energy?
Note that this has nothing to do with angular momentum, as the angular
momentum of something else must have been increased the exact same amount as
the momentum of the spinning object was reduced.
Imagine a spinning ball in space, if you grabbed it in your hand (thus
stopping it spinning and creating some friction heat) then the overall
momentum would remain constant - ie you'd start spinning a bit. No momentum
has been lost, that is impossible without any external force (which you
don't have if you're both floating in space). Some of the rotational energy
has been converted to heat though. Do the sums, rotational energy is
1/2*I*w^2 (I is moment of inertia, w is angular velocity in radians per
second), and angular momentum is I*w. You will see that during the
"collision", the energy is reduced even though the momentum remains
constant.
> Two objects with no angular momentum at all collide off-center, and
> they get stuck to each other. The resulting union of masses will start
> spinning because of the collision.
A very good example of conservation of angular momentum... You do realise
that two objects travelling in a straight line "off-center" have non-zero
angular momentum as a system?
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And lo on Thu, 01 Nov 2007 17:49:40 -0000, Nicolas Alvarez
<nic### [at] gmailisthebestcom> did spake, saying:
>> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
>>
>>> Then wikipedia is horribly wrong, I suppose.
>> Wikipedia isn't always right....
>
> Then edit it.
>
> I hate people saying "don't use wikipedia because it's wrong". Make it
> right instead of whining!
Except of course when you correct it and someone then reverts your
changes, then you revert it back, and then they revert it, and so on and
so forth until you get fed-up and the wrong information just stays there.
--
Phil Cook
--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com
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On Thu, 01 Nov 2007 14:49:40 -0300, Nicolas Alvarez wrote:
> Jim Henderson escribió:
>> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
>>
>>> Then wikipedia is horribly wrong, I suppose.
>>
>> Wikipedia isn't always right....
>
> Then edit it.
I'm unwilling to go through the process necessary to set an account up
and become recognised as having good enough information for them. And
with regards to this particular topic, I'm unqualified to submit an
expert opinion on the subject - however, what I recall from my college
courses, the information that's there is not 100% accurate. It would
take me research time and energy (neither of which I have, though I *do*
have mass ;) ) to sufficiently correct the content.
> I hate people saying "don't use wikipedia because it's wrong". Make it
> right instead of whining!
Um, that's not what I said. I do in fact use Wikipedia quite a bit, and
even consider many of the articles to contain sufficient information to
be authoritative on the subject they're about.
At the same time, people pointing to Wikipedia as the end-all be-all of
all knowledge and wisdom is equally as ridiculous.
Jim
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Jim Henderson escribió:
> On Thu, 01 Nov 2007 14:49:40 -0300, Nicolas Alvarez wrote:
>> Then edit it.
>
> I'm unwilling to go through the process necessary to set an account up
> and become recognised as having good enough information for them.
I have done many edits anonymously (most minor, like typos or fixing
links to disambiguation pages). At least, leave a message on the talk
page saying something is wrong.
>> I hate people saying "don't use wikipedia because it's wrong". Make it
>> right instead of whining!
>
> Um, that's not what I said.
I didn't say *you* did, but I heard people saying that many times.
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On Sat, 03 Nov 2007 19:12:42 -0300, Nicolas Alvarez wrote:
> Jim Henderson escribió:
>> On Thu, 01 Nov 2007 14:49:40 -0300, Nicolas Alvarez wrote:
>>> Then edit it.
>>
>> I'm unwilling to go through the process necessary to set an account up
>> and become recognised as having good enough information for them.
>
> I have done many edits anonymously (most minor, like typos or fixing
> links to disambiguation pages). At least, leave a message on the talk
> page saying something is wrong.
There again, I don't have the personal expertise to provide specific
examples - nor the time (I need about another 10 hours per day just to do
the things I'm behind on) or the energy to keep up with the discussion.
>>> I hate people saying "don't use wikipedia because it's wrong". Make it
>>> right instead of whining!
>>
>> Um, that's not what I said.
>
> I didn't say *you* did, but I heard people saying that many times.
Well, in the context of this discussion, it sounded like you were
directing the comment at me.
Jim
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> > Jim Henderson escribió:
> >> On Thu, 01 Nov 2007 14:49:40 -0300, Nicolas Alvarez wrote:
> >>> Then edit it.
> >>
> >> I'm unwilling to go through the process necessary to set an account up
> >> and become recognised as having good enough information for them.
I tried to work on wikipedia.
They say you must have verifiable citations is all. And even though I provided
citation from federation proceedinge (experimental biology) they would not
accept it.
I am not going to into details of the event. But I was convinced that they were
no less than puppets for an evil pharmaceutical industry, and had to give up
trying disseminate real information thru wiki.
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